1. Field of the Invention
The present invention relates to a fiber optic gyroscope sensing coil and to a method of manufacture same. More particularly, this invention pertains to improved fiber optic sensing coils and a method for their formation.
2. Description of the Art
Fiber optic gyroscopes comprise an interferometer which includes a light source, beam splitter and detector, and a fiber optic sensing coil. Light from the light source is split by the beam splitter and applied to the ends of the sensing coil. The interferometer and associated electronics process the phase relationship between the two interfering, counter-propagating beams of light that emerge from opposite ends of the coil. The difference between the phase shifts experienced by the two beams provides a measure of the rate of rotation of the platform to which the instrument is fixed.
One of the problems associated with presently known fiber optic gyroscope sensing coils is their temperature sensitivity. It is therefore common practice to wind fiber onto a coil such that the fiber is laid up in alternate layers, supplied from alternate windings of fiber. Such an arrangement is illustrated in FIG. 1 and helps to reduce the effect of any radial temperature difference experienced by clockwise and counter-clockwise light beams. Any temperature change experienced in the path length will change the time it takes for the light to travel around the coil and will have an adverse effect on the performance of a gyroscope employing such coils. The above mentioned arrangement, whilst reducing the temperature sensitivity is unable to average out temperature effects on fiber lengths shorter than the double layer as this fiber comes from only one of the two spools. In addition to the above, any axial temperature variation dT/dx along the axis X of the sensing coil will also have a significant effect on the performance of the gyro as such temperature variations cannot be compensated for.
Referring to FIG. 1 it will be appreciated that the fiber at the end of each layer will have to undergo a double layer Jump to the next layer before it is again wound around the coil. Such Jumping can result in microbending with the associated, and undesirable, consequence of polarization cross coupling. Such cross coupling is known to be one of the key error sources of the gyro.
U.S. Pat. No. 4,856,900 discloses a quadrupole-wound sensing coil for a fiber optic gyroscope in which alternate double layers of fiber are wound onto a coil and microbends are reduced by winding the fiber through three concentric turns adjacent distinct home flanges whilst the alternate double layer is being wound. A new alternate double layer is then easily started without bending the fiber, thereby avoiding the undesirable effects of microbends.
The above mentioned approach, whilst reducing the problems associated with microbends, does little to reduce the problems associated with axial and radial temperature PG,4 sensitivity. Indeed, the use of such double layers of fiber would exacerbate the radial temperature sensitivity problem.